This invention relates to the field of processing beta-phase nickel/titanium-base alloys and, more particularly, to the field of processing beta-phase nickel/titanium-base, shape-memory alloys.
Materials, both organic and metallic, capable of possessing shape memory are well known. An article made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. The article is said to have shape memory for the reason that, upon the application of the heat alone, it can be caused to revert or attempt to revert from its heat-unstable configuration to its original, heat-stable configuration, i.e., it "remembers" its original shape.
Among metallic alloys the ability to possess shape memory is a result of the fact that the alloy undergoes a reversible transformation from an austenitic state to a martensitic state with a change of temperature. Also, the alloy is considerably stronger in its austenitic state than in its martensitic state. This transformation is sometimes referred to as a thermoelastic martensitic transformation. An article made from such an alloy, for example, a hollow sleeve, is easily deformed from its original configuration to a new configuration when cooled below the temperature at which the alloy is transformed from the austenitic state to the martensitic state. The temperature at which this transformation begins is usually referred to as M.sub.s and the temperature at which it finishes M.sub.f. When an article thus deformed is warmed to the temperature at which the alloy starts to revert back to austenite, referred to as A.sub.s (A.sub.f being the temperature at which the reversion is complete), the deformed object will begin to return to its original configuration.
Alloys of nickel and titanium have been demonstrated to have shape-memory properties which render them highly useful in a variety of applications.
Shape-memory alloys have found use in recent years in, for example, pipe couplings (such as are described in U.S. Pat. Nos. 4,035,007 and 4,198,081 to Harrison and Jervis), electrical connectors (such as are described in U.S. Pat. No. 3,740,839 to Otte and Fischer), switches (such as are described in U.S. Pat. No. 4,205,293), actuators, etc., the disclosures of which are incorporated hereby by reference.
Notwithstanding the obvious utility of shape-memory alloys, the forming of parts from shape-memory alloys present certain difficulties. Some of the shape-memory alloys, such as those illustrated in U.S. Pat. No. 4,283,233 to Goldstein et al. may be readily cold worked followed by a warm anneal. Other alloys, such as those found in U.S. Pat. No. 3,753,700 to Harrison et al., are subject to serve embrittlement when cold worked. These latter alloys are usually hot worked followed by hot anneal. An alternative treatment of these latter alloys would be working at liquid-nitrogen temperatures to take advantage of the increased ductility of the martensitic phase. Needless to say, such a treatment is impractical.
In the typical prior uses of shape-memory alloys, the deformed object is allowed to begin reversion to its original configuration without being restrained by a force of any great amount. For example, in the pipe couplings of the aforementioned U.S. Pat. Nos. 4,035,007 and 4,198,001, the coupling when heated is allowed to freely contract until constrained by the external dimensions of the pipe.
It has been found, however, that the amount of motion of the heated, recoverable member is drastically reduced when a restraining load is applied. With increasing load, the amount of motion at recovery is correspondingly reduced. At some amount of applied load, the amount of motion will be effectively zero. In other words, the amount of work that is obtainable from any recoverable member is reduced as the restraining load is increased.
It would be desirable to increase the work obtainable from any recoverable member.
Thus it is an object of the invention to increase the amount of work that can be obtained from a heat-recoverable, shape-memory alloy member when it is subject to restraint by an applied force.
It is another object of the invention to increase the amount of force that can be obtained from a rigidly restrained, heat-recoverable member by a method that is practically feasible.
It is still another object of the invention to process an alloy having limited cold ductility by a method that is practically feasible.
It is a further object of the invention to manufacture an article by this method.
These and other objects of the invention will become apparent to those skilled in the art after considering the following description in conjunction with the accompanying drawings.